Can your DS4 listen to a chapter length audiobook? I read my DD and DS their first novel length book when DD was 4. We read "Tales of a Fourth Grade Nothing" and they were both spellbound. It's about a mischievous little boy and very funny so your DS might be interested. Also I would suggest Mrs. Piggle Wiggle books. The chapters are stand alone vignettes and also pretty funny and entertaining.
If you have time you could buy audiobooks on a CD but if you don't there is a website called Audiobook.com that offers downloads. Not sure how it works (where does it go? what can play it? etc) but this could be done immediately.
If your kids love to have books read to them this could be a great solution. Last summer we listened to 3/4 of "Fellowship of the Ring" on a 15 hour drive and had almost no whining or bickering.
